Flexible unbonded pipes as such are well known in the art. Such pipes comprise an inner liner which forms a barrier against the outflow of the fluid which is conveyed through the pipe, and one or more armouring layers on the outer side of the inner liner (Outer armouring layer(s)). The flexible pipe may comprise additional layers such as one or more inner armour layers to prevent the collapse of the inner liner. Such inner armouring layer or layers are normally referred to as a carcass. An outer sheath may be provided with the object of forming a barrier against the ingress of fluids from the pipe surroundings to the armour layers.
Typical unbonded flexible pipes are e.g. disclosed in WO0161232A1, U.S. Pat. No. 6,123,114 and U.S. Pat. No. 6,085,799.
The term “unbonded” means in this text that at least two of the layers including the armouring layers and polymer layers are not bonded to each other. In practice the pipe will comprise at least two armouring layers, which are not bonded to each other directly or indirectly via other layers along the pipe. Thereby the pipe becomes bendable and sufficiently flexible to roll up for transportation.
The above-mentioned type of flexible pipes is used, among other things, for off shore as well as some on-shore applications for the transport of fluids and gases. Flexible pipes can e.g. be used for the transportation of fluids where very high or varying water pressure exists along the longitudinal axis of the pipe, such as riser pipes which extend from the seabed up to an installation on or near the surface of the sea, pipes for transportation of liquid and gases between installations, pipes which are located at great depths on the seabed, or between installations near the surface of the sea.
In traditional flexible pipes, the one or more outer armouring layers are most often in the form of helically wound steel wires e.g. shaped as profiles, where the individual layers may be wound at different winding angle relative to the pipe axis.
When using such prior art flexible pipes for transportation of aggressive gases, raw oils and similar fluids, it has been found that undesired and often corrosive gases and liquids are diffusing through the inner liner and into the outer armouring layers.
In prior art pipes, this problem has been solved in a number of different ways. In EP 1119684 a solution is disclosed where the lumen between an inner liner and an outer sheath can be flushed to remove undesired gases and liquids that has been diffusing through the inner liner into the lumen. This solution is however not suitable in all situations.
Another approach for preventing the aggressive and destructive corrosion of the armour wires is to provide armour wires of a non-corrosive material. Such solution has been disclosed in WO 02095281, wherein the armouring wires are made from a composite material. Such wires are however relatively expensive.
In WO0231394 it is suggested to allow environmental sea water to come into contact with the outer armour layers whereby gases and liquids that have been diffusing through the inner liner are flushed away from the armours. As the sea water may be corrosive in itself, this solution either requires that the armour wires are of a non-corrosive material or that the pipe is used in low corrosive outer environments.
U.S. Pat. No. 6,006,788 discloses a flexible pipe with an internal gas proof undulating metal tube. However, this pipe is relatively stiff as the internal gas proof undulating metal tube needs to have a certain thickness in order to be sufficiently stable. Furthermore, the internal gas proof metal tube need to be resistive to the medium to be transported in the pipe, which in practice means that pipes of this type have a very limited used.